Solid electrolytic capacitor and process for fabricating same

ABSTRACT

The invention provides a solid electrolytic capacitor wherein the anode has a dielectric oxide film of a structure less susceptible to damage due to mechanical stresses and which is diminished in leakage current and less prone to short-circuiting, and a process for fabricating the capacitor. The capacitor of the invention comprises an anode of aluminum having a dielectric oxide film formed over a surface thereof from amorphous alumina, and is characterized in that a plurality of tunnel-shaped etching pits are formed in the anode. The process of the invention for fabricating the solid electrolytic capacitor includes the steps of forming a plurality of tunnel-shaped etching pits in an aluminum material, effecting anodic oxidation by immersing the aluminum material in an electrolytic solution containing oxalic acid or the like, and effecting anodic oxidation by immersing the aluminum material in an electrolytic solution containing boric acid or like inorganic acid or a salt thereof or containing adipic acid or like organic acid or a salt thereof and applying a voltage at least three times the rated voltage of the capacitor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 11/348,329, filed Feb. 7,2006, which is based upon and claims the benefit of priority from theprior Japanese Patent Application No. 2005-35579, filed Feb. 14, 2005,the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to solid electrolytic capacitorscomprising an aluminum anode and a process for fabricating thecapacitor.

BACKGROUND ART

Solid electrolytic capacitors comprising a TCNQ complex salt,polypyrrole or like high polymer serving as a solid electrolyte and ananode of aluminum foil, thin sheet or the like have found wide use invarious electronic devices. There is in recent years a rapidly growingdemand for solid electrolytic capacitors having higher voltageresistance. With such solid electrolytic capacitors, a dielectric oxidefilm or layer is formed over the surface of the anode as is well known.As a method of giving increased voltage resistance to solid electrolyticcapacitors, i.e., to the dielectric oxide film thereof, it is practiceto increase the voltage (formation voltage) to be applied to thealuminum material for use as an anode or to be made into an anode in theanodic oxidation step of forming the dielectric oxide film. In thecommon conventional process for fabricating solid electrolyticcapacitors, the formation voltage for anodic oxidation is set at a levelapproximately three times the rated voltage of the capacitor.

However, if a voltage higher than this value is used for anodicoxidation, the solid electrolytic capacitor obtained has the problem ofbecoming susceptible to marked leakage current and to short-circuitfailure. To overcome this problem, it is practice to form on the anode adielectric oxide film from amorphous alumina instead of crystallinealumina (see, for example, JP 5-343267A). The dielectric oxide film ofcrystalline alumina undergoes volumetric shrinkage during formation todevelop defects, whereas dielectric oxide film of amorphous aluminaremains almost free of volumetric shrinkage during formation and isgreatly diminished in defects. The leakage current or short-circuitfailure of solid electrolytic capacitors is attributable to the defectsin the dielectric oxide film, so that the formation of the dielectricoxide film from amorphous alumina provides solid electrolytic capacitorshaving high voltage resistance, diminished in leakage current and lessprone to short-circuit failure.

The aluminum material to be used as or made into an anode, and the anodeare subjected to a bending stress, tensile stress and like mechanicalstresses (physical stresses) in the process for fabricating solidelectrolytic capacitors. For example, in the case of solid electrolyticcapacitors of the rolled-up type, a dielectric oxide film is formed onaluminum foil having a large width and to be made into anodes, followedby a cutting step, in which the aluminum foil of large width is cut intoseparate pieces of aluminum foil of reduced width, namely, into separateanodes. After the cutting step, a lead tab is joined to the anode bycrimping, and the anode is connected to a lead wire by the lead tabterminal. The anode is then rolled up along with a cathode and separatorpaper to make a capacitor element.

If the anode is subjected to a mechanical stress in the cutting step,joining step or rolling-up step described above, the dielectric oxidefilm on the anode will be thereby injured to develop defects anew. Ifthe defects thus subsequently occurring result in increased leakagecurrent and more serious short-circuit failure in the solid electrolyticcapacitor, the advantage of the dielectric oxide film of amorphousalumina becomes impaired. The present invention, which has overcome suchproblems, provides a solid electrolytic capacitor wherein the anode isprovided with a dielectric oxide film of a structure less susceptible todamage or faults due to a mechanical stress and which is smaller inleakage current and less prone to short-circuit failure thanconventional solid electrolytic capacitors, and a process forfabricating the capacitor.

SUMMARY OF THE INVENTION

The present invention provides a solid electrolytic capacitor comprisingan anode of aluminum having a dielectric oxide film formed over asurface thereof from amorphous alumina, the solid electrolytic capacitorbeing characterized in that a plurality of tunnel-shaped etching pitsare formed in the anode.

The present invention also provides a process for fabricating a solidelectrolytic capacitor including the steps of forming a plurality oftunnel-shaped etching pits in an aluminum material for use as an anodeof the capacitor by etching the aluminum material, effecting anodicoxidation by immersing the aluminum material in an electrolytic solutioncontaining oxalic acid, phosphoric acid, sulfuric acid or the like, andeffecting anodic oxidation by immersing the aluminum material in anelectrolytic solution containing boric acid, phosphoric acid or likeinorganic acid or a salt thereof or in an electrolytic solutioncontaining adipic acid, azelaic acid or like organic acid or a saltthereof and applying a voltage at least three times the rated voltage ofthe capacitor.

The invention provides another process for fabricating a solidelectrolytic capacitor including the steps of forming a plurality oftunnel-shaped etching pits in an aluminum material for use as an anodeof the capacitor by etching the aluminum material, and effecting anodicoxidation by immersing the aluminum material in an electrolytic solutioncontaining boric acid, phosphoric acid or like inorganic acid or a saltthereof or in an electrolytic solution containing adipic acid, azelaicacid or like organic acid or a salt thereof at a temperature of up to50° C. without subjecting the aluminum material to a hydration step andapplying a voltage at least three times the rated voltage of thecapacitor.

The tunnel-shaped etching pits formed in the anode enable the anode tohave a larger remaining aluminum portion than etching pits whichresemble pores of common sponge, effectively diffusing the mechanicalstress acting on the aluminum material and the anode during thefabrication process. With the solid electrolytic capacitor of theinvention, the dielectric oxide film is consequently less susceptible todamage or faults in the fabrication process, rendering the capacitorsmaller in leakage current and less prone to short-circuiting thanconventional like capacitors. The capacitor fabrication processdescribed provides solid electrolytic capacitors which are reduced inleakage current. The term “aluminum material” includes aluminum foil ora thin aluminum sheet to be used as or to be made into an anode.

Experiments conducted by the present inventor have revealed that theadvantage of the invention can be realized remarkably when the ratio ofetching pits having a diameter of 0.5 to 1.5 μm to all etching pits ofthe anode is at least 50%. Further it is also revealed that theadvantage of the invention can be realized remarkably when the ratio ofspacings of 0.5 to 1.0 μm between respective adjacent pairs of etchingpits to all spacings between respective adjacent pairs of etching pitsis at least 50%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a solid electrolytic capacitor of therolled-up type embodying the invention;

FIG. 2 is a perspective view of a capacitor element of the capacitorembodying the invention; and

FIG. 3 is a diagram schematically showing in section aluminum foil foruse in fabricating the solid electrolytic capacitor of the rolled-uptype embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described below with reference to anembodiment thereof. FIG. 1 is a sectional view of the embodiment of theinvention, i.e., a solid electrolytic capacitor of the rolled-up type,and FIG. 2 is a perspective view of a capacitor element 1 constitutingthe capacitor. The capacitor element 1 is disposed inside a bottomedtubular metal case 3. (FIG. 2 shows the capacitor element 1 before it isenclosed in the case 3.) The capacitor element 1 is generally in theform of a cylinder and is fabricated by winding an anode 5 and a cathode7, each in the form of a strip of aluminum foil and, into a roll withseparator paper 9 provided therebetween. (FIG. 2 shows the anode 5,separator paper, etc. as unwound and indicated in broken lines.) Theanode 5 and the cathode 7 are connected to an anode lead wire 13 and acathode lead wire 15 by lead tab terminals 11, 11, respectively. Thespace between the anode 5 and the cathode 7 is filled with a solidelectrolyte layer (not shown) of TCNQ complex salt or electricallyconductive high polymer. The conductive high polymer to be used is, forexample, polypyrrole, polyfuran, polyaniline or the like.

Provided above the roll of capacitor element 1 is a sealing packing 17of rubber (such as butyl rubber). The metal case 3 is locallyconstricted by drawing so as to compress the packing 17 and curled atits upper end. A seat plate 19 of insulating resin is so disposed as toclose the opening of the metal case 3 and provided with an anodeterminal 21 and a cathode terminal 23 thereon. In fabricating thecapacitor, the packing 17 is provided on the roll of capacitor element1, and the seat plate 19 is provided over the packing, with the leadwires 13, 15 so positioned as shown in FIG. 2. The lead wires 13, 15extend through the packing 17 and the seat plate 19 to project from theupper surface of the seat plate 19. The projecting upper ends are eachpressed into a thin plate, and the lead wires 13, 15 are then bent,whereby the anode terminal 21 and the cathode terminal 23 are arrangedover the seat plate 19 as shown in FIG. 1.

The solid electrolytic capacitor of the invention is fabricated by theprocess to be described below in the case where a conductive highpolymer is used as the solid electrolyte. The solid electrolyticcapacitor of the invention is characterized by the construction of theanode 5. The anode 5 is made by treating and machining a strip ofaluminum foil having a large width. The strip is prepared and thenetched first. This etching step is performed to form a multiplicity oftunnel-shaped etching pits in opposite surfaces of the foil(approximately perpendicular to the foil surface). The particulars ofthe etching step are not limited specifically insofar as the object ofthe invention can be fulfilled, and advantage thereof can be ensured.

For example, the etching step includes a first stage of forming initialpits and a second stage of enlarging these pits. In the first stage, dccurrent is passed through the aluminum foil for dc etching using anaqueous solution of hydrochloric acid serving as an etchant andcontaining an acid such as oxalic acid, phosphoric acid or sulfuric acidadded thereto. The length (depth), density and diameter of the etchingpits to be formed are controlled by adjusting the current density andthe amount of electricity.

After the completion of the first stage, the aluminum foil is washed andsubjected to the second stage, in which the aluminum foil is immersed inan aqueous solution of sulfuric acid, nitric acid or the like tochemically dissolve the foil, whereby the pits are enlarged in diameter.The second stage may be followed by the hydration step of immersing thealuminum foil in pure water. When performed, the hydration step formsaluminum hydroxide on the surface of the aluminum foil. This results inpromoted formation of an alumina layer and reduced power consumption inthe anodic oxidation step to be described next.

Anodic oxidation is conducted after the etching step or hydration stepto form on the surface of the aluminum foil a dielectric oxide filmconsisting essentially of amorphous alumina. For example, the anodicoxidation step includes a first stage and a second stage. In the firststage, the aluminum foil is immersed in an electrolytic solutioncontaining an acid such as oxalic acid, phosphoric acid, sulfuric acidor the like and an anodic oxidation treatment is effected at apredetermined current density. The first stage performed forms a porousamorphous alumina layer (anodized layer of aluminum) over the aluminumfoil. It is desirable to select the current density from the range of 10to 1000 mA/cm².

In the second stage, the aluminum foil is immersed in an electrolyticsolution containing boric acid, phosphoric acid or like inorganic acidor a salt thereof, or in an electrolytic solution containing adipicacid, azelaic acid or like organic acid or a salt thereof and then ananodic oxidation treatment is effected at a predetermined currentdensity and a specified raised voltage. The electrolytic solutioncontaining the inorganic acid may further contain a salt thereof addedthereto, or when containing the organic acid, the electrolytic solutionmay further contain a salt thereof. Consequently, amorphous aluminagrows from the pit bottom portions of the porous amorphous alumina layerformed by the first stage so as to fill the pits. Preferably, the raisedvoltage (formation voltage) to be applied is at least three times therated voltage of the solid electrolytic capacitor to be fabricated. Thecurrent density is selected preferably from the range of 10 to 1000mA/cm².

Instead of the first stage and the second stage, the anodic oxidationstep may be performed alternatively by immersing the aluminum materialin an electrolytic solution containing boric acid, phosphoric acid orlike inorganic acid or a salt thereof, or in an electrolytic solutioncontaining adipic acid, azelaic acid or like organic acid or a saltthereof and passing current at a predetermined current density and aspecified raised voltage. In the case where the anodic oxidation step isperformed in this way, the hydration step is not performed. Theelectrolytic solution may contain a salt of the inorganic acid inaddition to the acid or a salt of the organic acid in addition to thisacid. The electrolytic solution is preferably up to 50° C. intemperature. It is desirable that the formation voltage be at leastthree times the rated voltage, and that the current density be selectedfrom the range of 10 to 500 mA/cm².

FIG. 3 is a diagram schematically showing in section the aluminum foilresulting from the anodic oxidation step, namely, after a dielectricoxide film has been formed on the foil. The aluminum foil to be madeinto the solid electrolytic capacitor of the rolled-up type of theinvention is preferably 50 to 200 μm in thickness. The tunnel-shapedetching pits to be formed by the etching step are given such a diameterthat the pits will not be closed even when the anodic oxidation step isformed at a high formation voltage. To ensure the advantage of thepresent invention, it is desired that at least 50% of the etching pitswhich are formed after the formation of dielectric oxide film have adiameter of 0.5 to 1.5 μm as will be described below. Further when eachadjacent pair of etching pits are spaced apart by a certain distance,the remaining portion of the aluminum foil (the portion remaining afterthe etching step) will be greater, whereby the damage to the dielectricoxide film due to a mechanical stress can be suppressed moreeffectively. To ensure the advantage of the invention, therefore, it isdesired that at least 50% of the spacings (edge-to-edge distances)between respective adjacent pairs of etching pits which are formed afterthe formation of dielectric anodic oxide film be 0.5 to 1.0 μm as willbe described below. Preferably, the pits have a length less than half ofthe thickness of the aluminum foil as shown in FIG. 3. Furthermore, itis desired that the proportion of crystalline alumina in the dielectricoxide film be made small to the greatest possible extent. Stated morespecifically, the dielectric oxide film formed on the aluminum foil ispreferably up to 5% in the degree of crystallinity (mass fraction degreeof crystallinity).

The etching step and the anodic oxidation step are followed by the stepof cutting the aluminum foil to complete strips of anode 5 for use incapacitor elements 1. According to embodiments of the invention,aluminum foil is used also for the cathode 7. Like the anode foil,strips of cathode 7 are prepared by the step of etching, anodicoxidation step and cutting step. According to the present invention,however, the construction or structure of the cathode 7 is not limitedparticularly; the etching step and the oxidation step described aboveneed not always be performed for making the cathode 7. For example, acetching may be conducted to form in aluminum foil etching pitsresembling those of sponge, or a dielectric oxide film comprisingcrystalline alumina may be formed on aluminum foil.

The anode 5 and the cathode 7 prepared are used for fabricating thecapacitor element 1 shown in FIG. 2. First, a lead tab terminal 11provided at one end of an anode lead wire 13 is joined to the anode 5 bycrimping to connect the anode lead wire 13 to the anode 5. At the sametime, a cathode lead wire 15 is connected to the cathode 7 by anotherlead tab terminal 11. The anode 5 and the cathode 7 are thereafter woundinto a roll, with separator paper 9 provided therebetween. A holdingtape 25 is provided around the roll of capacitor element 1.

The anode 5 and the cathode 7 are each made by cutting aluminum foil andtherefore have cut ends which are not covered with any dielectric oxidefilm over the end faces. Accordingly, a dielectric oxide film is formedby a chemical conversion step over the cut end faces of the capacitorelement 1 made as shown in FIG. 2. The capacitor element 1 is furtherheat-treated at a temperature of 150 to 300° C.

The heat treatment step is followed by the step of forming a conductivehigh polymer layer between the anode 5 of the capacitor element 1 andthe cathode 7 thereof. This step is performed, for example, by placing amonomer, which is polymerizable into a conductive high polymer, into anoxidizer solution (such as an alcohol solution of ferricp-toluenesulfonate) to obtain a solution, impregnating the capacitorelement 1 with the monomer solution and thermally polymerizing themonomer. The capacitor element 1 and a sealing packing 17 are thereafterplaced into a metal case 3, which is then locally constricted by drawingand curled, followed by aging. A seal plate 19 is then provided, and thelead wires 13, 15 are finally machined to complete a solid electrolyticcapacitor as shown in FIG. 1.

Solid electrolytic capacitors (10 mm in diameter and 8.0 mm in length),50 V in rated voltage, were fabricated according to the invention. Solidelectrolytic capacitors were similarly fabricated according to the priorart. The capacitors were then checked for characteristics forcomparison. Twenty samples were fabricated in each of Examples andComparative Examples with the results described below.

Example 1

Solid electrolytic capacitors were prepared in the following manner inthis example. Anodes 5 were made by treating and machining aluminum foilhaving a thickness of 100 μm. In the first stage of an etching step, thealuminum foil was subjected to dc etching in an aqueous solution ofhydrochloric acid containing sulfuric acid at a current density of 50mA/cm² for 5 minutes. In the second stage, the aluminum foil wasimmersed in 5 wt. % aqueous solution of nitric acid at 50° C. andchemically dissolved to enlarge the etching pits. Subsequent to theetching step, an anodic oxidation step was performed without conductinghydration. In the first stage of this step, the aluminum foil wasimmersed in an electrolytic solution of 5 g/L of oxalic acid andsubjected to anodic oxidation at a current density of 200 mA/cm² for 7minutes. In the second stage, the aluminum foil was immersed in anelectrolytic solution of 1.0 g/L of ammonium pentaborate and held at araised voltage of 150 V (three times the rated voltage 50V) for 15minutes at a current density of 200 mA/cm². Consequently, in all the 20samples, tunnel-shaped pits were formed in the anode 5, wherein theratio of etching pits having a diameter of 0.5 to 1.5 μm and the ratioof spacings of 0.5 to 1.0 μm between respective adjacent pairs of pitsis at least 50%.

The pits were checked for diameter and pit-to-pit spacing under ascanning electron microscope, and the ratios were calculated from theresults.

Further in all the 20 samples, the dielectric oxide film of the anode 5was up to 5% in the degree of crystallinity to find that the film wascomposed of amorphous alumina almost entirely. The degree ofcrystallinity was determined by checking the dielectric oxide film byX-ray diffractometry and calculating the ratio of the measurementrelative to an authentic sample which was composed of 100% ofgamma-alumina. The capacitor element 1 was heat-treated at a temperatureof 250° C. The solid electrolytic layer of the capacitor element 1 wasprepared by thermal polymerization using 3,4-ethylenedioxythiophene as amonomer and an alcohol solution of ferric p-toluenesulfonate as anoxidizer solution. The cathode 7 was made by subjecting aluminum foil toac etching in an aqueous hydrochloric acid solution containing sulfuricacid. The particulars concerning the cathode 7 and not mentioned are thesame as those already described as to the anode 5.

Example 2

Solid electrolytic capacitors were fabricated in the same manner as inExample 1 except that the formation voltage applied in the second stageof the anodic oxidation step was 250 V (five times the rated voltage50V), with a longer period of time taken for the rise of voltage than inExample 1. In all the 20 samples of Example 2, tunnel-shaped pits wereformed in the anode 5, wherein at least 50% of the tunnel-shaped pitshad a diameter of 0.5 to 1.5 μm is at least 50% and at least 50% of thespacings between respective adjacent pairs of pits were 0.5 to 1.0 μm.All the 20 samples were also found to be up to 5% in the degree ofcrystallinity of the dielectric oxide film.

Example 3

Solid electrolytic capacitors were fabricated in the same manner as inExample 1 except that the formation voltage applied in the second stageof the anodic oxidation step was 450 V (nine times the rated voltage),with a longer period of time taken for the rise of voltage than inExample 2. In all the 20 samples of Example 3, tunnel-shaped pits wereformed in the anode 5, wherein at least 50% of the tunnel-shaped pitshad a diameter of 0.5 to 1.5 μm and at least 50% of the spacings betweenrespective adjacent pairs of pits were 0.5 to 1.0 μm. All the 20 sampleswere also found to be up to 5% in the degree of crystallinity of thedielectric oxide film.

Examples 1 to 3 are different in the formation voltage of the secondstage of the anodic oxidation. For all the solid electrolytic capacitorsof these examples, the ratio of tunnel-shaped pits having a diameter of0.5 to 1.5 μm is at least 50% and the ratio of spacings of 0.5 to 1.0 μmbetween respective adjacent pairs of pits is at least 50%. This featureis attributable to the fact that the second stage of the anodicoxidation step causes amorphous alumina to grow from the pit bottomportions of the porous amorphous alumina layer formed by the first stageso as to fill the pits as already stated, with the result thatdifferences in formation voltage produce no influence on the thicknessof the dielectric oxide film

Example 4

Solid electrolytic capacitors were fabricated in this example bysubjecting aluminum foil to dc etching in an aqueous hydrochloric acidsolution containing sulfuric acid at a current density of 75 mA/cm² for5 minutes and thereafter immersing the foil in an aqueous solution of 5wt. % of nitric acid at 50° C. to chemically dissolve the foil.Consequently, in all the 20 samples, tunnel-shaped pits formed in theanode 5, wherein at least 50% of the tunnel-shaped pits had a diameterof 0.5 to 1.5 μm and less than 40% of the spacings between respectiveadjacent pairs of pits were 0.5 to 1.0 μm. With the exception of thisfeature, the solid electrolytic capacitors obtained were the same asthose fabricated in Example 2.

Example 5

Solid electrolytic capacitors were fabricated in this example bysubjecting aluminum foil to dc etching in an aqueous hydrochloric acidsolution containing sulfuric acid at a current density of 50 mA/cm² for5 minutes and thereafter immersing the foil in an aqueous solution of 2wt. % of nitric acid at 50° C. to chemically dissolve the foil.Consequently, in all the 20 samples, tunnel-shaped pits formed in theanode 5, wherein less than 40% of the tunnel-shaped pits had a diameterof 0.5 to 1.5 μm and at least 50% of the spacings between respectiveadjacent pairs of pits were 0.5 to 1.0 μm. With the exception of thisfeature, the solid electrolytic capacitors obtained were the same asthose fabricated in Example 2.

Comparative Example 1

Solid electrolytic capacitors were fabricated in this comparativeexample by immersing aluminum foil in an electrolytic solution of 0.5g/L of ammonium adipate at 85° C. after the completion of a hydrationstep, applying a raised voltage of 250 V to the solution at a currentdensity of 200 mA/cm² and thereafter holding this voltage for 15 minutesfor anodic oxidation. This step produced anodes 5 having a dielectricoxide film consisting substantially of crystalline alumina (80 to 95% inthe degree of crystallinity). With the exception of these features, thesolid electrolytic capacitors obtained were the same as those fabricatedin Example 2.

Comparative Example 2

Solid electrolytic capacitors were fabricated in this comparativeexample by subjecting aluminum foil to ac etching with sinusoidalcurrent (50 Hz) at a current density of 200 mA/cm² in an aqueoussolution containing hydrochloric acid and phosphoric acid, wherebyanodes 5 were made which had etching pits resembling pores of sponge.With the exception of these features, the solid electrolytic capacitorsobtained were the same as those fabricated in Example 2.

Comparative Example 3

Solid electrolytic capacitors were fabricated in this comparativeexample by performing an etching step in the same manner as inComparative Example 2, whereby anodes 5 were made which had etching pitsresembling pores of sponge. The formation voltage used was 150 V. Withthe exception of these features, the solid electrolytic capacitorsobtained were the same as those fabricated in Comparative Example 1.

Comparative Examples 4 and 5

Solid electrolytic capacitors were fabricated using a formation voltageof 250 V in Comparative Example 4 and using a formation voltage of 450 Vin Comparative Example 5. Otherwise, the capacitors were fabricated inthe same manner as in Comparative example 3.

TABLE 1 Specifications of anode Incidence Ratio of pits Ratio of pitFormation of short- Shape of having a diam. spacings of voltagecircuiting Film pits of 0.5-1.5 μm 0.5-1.0 μm (V) (%) Example 1Amorphous Tunnels At least 50% At least 50% 150 1.2 Example 2 AmorphousTunnels At least 50% At least 50% 250 0.0 Example 3 Amorphous Tunnels Atleast 50% At least 50% 450 0.0 Example 4 Amorphous Tunnels At least 50%Below 40% 250 2.1 Example 5 Amorphous Tunnels Below 40% At least 50% 2502.2 Comp. Ex. 1 Crystalline Tunnels At least 50% At least 50% 250 4.5Comp. Ex. 2 Amorphous Sponge — — 250 7.6 Comp. Ex. 3 Crystalline Sponge— — 150 18.3 Comp. Ex. 4 Crystalline Sponge — — 250 16.7 Comp. Ex. 5Crystalline Sponge — — 450 15.6

Table 1 shows the incidence of short-circuiting occurring in solidelectrolytic capacitors of Examples and Comparative Examples duringaging treatment which was conducted by applying a rated voltage of 50 Vto the capacitors. The table reveals that the capacitors of Examples 1to 5 wherein the anode 5 had tunnel-shaped etching pits and a dielectricoxide film of amorphous alumina are lower in the incidence ofshort-circuiting than those of Comparative Example 1 wherein the anodehad tunnel-shaped pits and a dielectric oxide film of crystallinealumina, and are also lower in the incidence of short-circuiting thanthose of Comparative Example 2 wherein the anode had pits resemblingsponge pores and a dielectric oxide film of amorphous alumina. Table 1also shows that the capacitors of Examples 1 to 5 are lower in theincidence of short-circuiting than those of Comparative Examples 3 to 5wherein the anode had pits resembling sponge pores and a dielectricoxide film of crystalline alumina.

Examples 1 to 3 indicate the capacitors of the invention are diminishedin the incidence of short-circuiting regardless of the formation voltagefor the anode 5. With attention directed to Examples 2 and 5 wherein theformation voltage was 250 V, it is understood that the capacitors of theinvention are less susceptible to short-circuiting when the ratio ofetching pits of the anode 5 having a diameter of 0.5 to 1.5 μm is atleast 50%. With attention directed to Examples 2 and 4 wherein theformation voltage was 250 V, it is seen that the capacitors of theinvention are less prone to short-circuiting if the ratio of spacings of0.5 to 1.0 μm between adjacent pairs of pits is at least 50%.

TABLE 2 Specifications of anode Initial electrical Ratio of pits Ratioof pit Formation characteristics Shape having a diam. spacings ofvoltage Cap. tan δ ESR LC Film of pits of 0.5-1.5 μm 0.5-1.0 μm (V) (μF)(%) (mΩ) (μA) Example 1 Amorphous Tunnels At least 50% At least 50% 1507.3 0.7 31 2.3 Example 2 Amorphous Tunnels At least 50% At least 50% 2503.4 0.6 36 1.1 Example 3 Amorphous Tunnels At least 50% At least 50% 4501.3 0.5 40 0.7 Example 4 Amorphous Tunnels At least 50% Below 40% 2503.8 0.6 35 3.6 Example 5 Amorphous Tunnels Below 40% At least 50% 2503.0 0.6 36 3.5 Comp. Ex. 1 Crystalline Tunnels At least 50% At least 50%250 6.9 0.5 34 13.2 Comp. Ex. 2 Amorphous Sponge — — 250 3.2 0.6 35 8.3Comp. Ex. 3 Crystalline Sponge — — 150 15.1 0.7 30 80.9 Comp. Ex. 4Crystalline Sponge — — 250 7.5 0.5 35 57.1 Comp. Ex. 5 CrystallineSponge — — 450 2.7 0.5 42 30.6

Table 2 shows the initial electrical characteristics of the solidelectrolytic capacitors of Examples and Comparative Examples. Theinitial electrical characteristics determined are capacitance (Cap.),tangent of loss angle (tan δ), equivalent series resistance (ESR) andleakage current (LC). Each of these values given in Table 2 is anaverage value of 20 samples. The values of capacitance and tangent ofloss angle are measurements at a frequency of 120 Hz. The value ofequivalent series resistance is a measurement at a frequency of 100 kHz.The value of leakage current is a value resulting from the applicationof a rated voltage of 50 V for 2 minutes.

Table 2 reveals that the solid electrolytic capacitors of Examples 1 to5 wherein the anode 5 had tunnel-shaped etching pits and a dielectricoxide film of amorphous alumina are greatly diminished in leakagecurrent as compared with the capacitors of Comparative Example 1 whereinthe anode had tunnel-shaped pits and a dielectric oxide film ofcrystalline alumina and with the capacitors of Comparative Example 2wherein the anode had pits resembling pores of sponge and a dielectricoxide film of amorphous alumina. Table 2 also indicates that thecapacitors of Examples 1 to 5 are much lower in leakage current thanthose of Comparative Examples 3 to 5 wherein the anode had pitsresembling pores of sponge and a dielectric oxide film of crystallinealumina. It will be understood from Tables 1 and 2 that the capacitorsof Examples 1 to 5 are comparable to those of Comparative Examples 1 to5 in tangent of loss angle and equivalent series resistance, and thatthe present invention has the advantage of inhibiting short-circuitingand diminishing leakage current without impairing these electricalcharacteristics.

With attention directed to Examples 1 to 3 listed in Table 2, it will beunderstood that the capacitors of the invention are reduced in leakagecurrent regardless of the formation voltage for the anode 5. Attentionfocused on Examples 2 and 5 wherein the same level of formation voltagewas used reveals that the invention produces a more remarkable effect todiminish leakage current when the ratio of pits having a diameter of 0.5to 1.5 μm is at least 50%. Attention given to Examples 2 and 4 whereinthe same level of formation voltage was used shows that the inventionproduces a more remarkable effect to diminish leakage current when theratio of spacings of 0.5 to 1.0 μm between respective adjacent pairs ofpits is at least 50%.

TABLE 3 Specifications of anode Ratio of pits Ratio of pit Formation BDVShape having a diam. spacings of voltage value Film of pits of 0.5-1.5μm 0.5-1.0 μm (V) (V) Example 1 Amorphous Tunnels At least 50% At least50% 150 63 Example 2 Amorphous Tunnels At least 50% At least 50% 250 84Example 3 Amorphous Tunnels At least 50% At least 50% 450 114 Comp. Ex.3 Crystalline Sponge — — 150 58 Comp. Ex. 4 Crystalline Sponge — — 25070 Comp. Ex. 5 Crystalline Sponge — — 450 83

Table 3 shows the BDV (breakdown voltage) values measured of the solidelectrolytic capacitors of Examples 1 to 3 and those of ComparativeExamples 3 to 5. The BDV value is a voltage value causing a dielectricbreakdown (short-circuiting) to the capacitor when the voltage to beapplied thereto is increased at a rate of 1 V/s at room temperature.With attention directed to Example 1 and Comparative Example 3 whereinthe formation voltage is 150 V, the BDV value (63 V) of Example 1 ishigher than the BDV value (58 V) of Comparative Example 3. With respectto Example 2 and Comparative Example 4 wherein the formation voltage is250 V, and to Example 3 and Comparative Example 5 wherein the formationvoltage is 450 V, the Example BDV values are higher than the ComparativeExample BDV values. These features reveal that the present inventionprovides solid electrolytic capacitors which are lower in the incidenceof short-circuiting and greatly diminished in leakage current and have ahigh BDV value, namely, high voltage resistance, regardless of theformation voltage. Thus, the present invention is extremely useful forfabricating solid electrolytic capacitors having a high rated voltage.

Although the present invention has been described above with referenceto solid electrolytic capacitors of the rolled-up type, the invention isapplicable to a wide variety of solid electrolytic capacitors comprisingan anode of aluminum, for example, to those wherein a single plate ofaluminum is used as the anode or to those of the stacked layer type. Theetching step or the anodic oxidation step described may be performedalternatively for a thin aluminum sheet having a large width and anincreased length. In the foregoing examples, pieces of aluminum foil ofa size suited to capacitor elements 1 (i.e., as cut into a suitablesize) may be subjected to the etching step or the anodic oxidation step.

The embodiment and examples given above are intended to illustrate thepresent invention and should not be construed as limiting the inventionset forth in the appended claims or reducing the scope thereof. Theconstruction of the capacitor of the invention is not limited to theabove embodiment but can be modified variously within the technicalscope defined in the claims.

1. A solid electrolytic capacitor comprising an anode of aluminum havinga dielectric oxide film formed over a surface thereof, the solidelectrolytic capacitor being characterized in that a plurality oftunnel-shaped etching pits are formed in the anode and the dielectricoxide film is formed from amorphous alumina, wherein the ratio ofetching pits having a diameter of 0.5 to 1.5 μm is at least 50%.
 2. Asolid electrolytic capacitor according to claim 1 wherein the dielectricoxide film is up to 5% in the degree of crystallinity.
 3. A solidelectrolytic capacitor according to claim 2 which is of the rolled-uptype and wherein the anode is aluminum foil.
 4. A solid electrolyticcapacitor according to claim 3 wherein the anode is 50 to 200 μm inthickness.
 5. A solid electrolytic capacitor according to claim 4wherein the ratio of spacings of 0.5 to 1.0 μm between respectiveadjacent pairs of etching pits is at least 50%.
 6. A solid electrolyticcapacitor according to claim 3 wherein the ratio of spacings of 0.5 to1.0 μm between respective adjacent pairs of etching pits is at least50%.
 7. A solid electrolytic capacitor according to claim 2 wherein theratio of spacings of 0.5 to 1.0 μm between respective adjacent pairs ofetching pits is at least 50%.
 8. A solid electrolytic capacitoraccording to claim 1 which is of the rolled-up type and wherein theanode is aluminum foil.
 9. A solid electrolytic capacitor according toclaim 8 wherein the anode is 50 to 200 μm in thickness.
 10. A solidelectrolytic capacitor according to claim 9 wherein the ratio ofspacings of 0.5 to 1.0 μm between respective adjacent pairs of etchingpits is at least 50%.
 11. A solid electrolytic capacitor according toclaim 8 wherein the ratio of spacings of 0.5 to 1.0 μm betweenrespective adjacent pairs of etching pits is at least 50%.
 12. A solidelectrolytic capacitor according to claim 1 wherein the ratio ofspacings of 0.5 to 1.0 μm between respective adjacent pairs of etchingpits is at least 50%.